An exceptional machine
The electrical part (about 70 tons) has a reluctor of 6 m in diameter and a dynamo, called exciter, placed in front of the wheel.

The exciter provides a continuous current which, through the medium of a collector (2 bronze bushings on the wheel axis) supplies 72 electro-magnets called inductors, placed around the driving wheel.
The 72 successive magnetic fields (alternated South Poles and North Poles) are put in motion by the rotation of the wheel. The variation of the magnetic fields in relation to 3 copper windings, alternated inside the fixed part (stator) surrounding the wheel, induces in these bobbins an alternative current of which the frequency varies according to the rotation speed of the wheel. It is this current which can be used if the machine was "in production".

The other part : the steam engine (about 100 tons), produces the mechanical force which operates the alternator. To place the steam engine in a favourable position at start-up, the turning gear (small steam engine with 2 V-type cylinders, located close to the wheel) has the job of slowly rotating the reluctor with 2 cog-wheels that the technician fits to the cog-rail of the big wheel for the starting up. The steam engine has 4 large cylinders connected 2 by 2 and each containing a piston.

These pistons are pushed by the steam which passes from the high-pressure cylinder (the smallest) into the intermediate-pressure cylinder, then into the 2 low-pressure cylinders at the same time.

The engine is said to be a treble expansion because the steam works as a 3-way system. The steam temperature at the entrance of the circuit is 200 degrees and its pressure is approximately 12 bars. The piston rod ends put in motion 2 rods which, with the help of the crankshaft, transforms the rectilinear motion of the steam engine into a fly-wheel rotation. Thanks to a leather strap, the wheel ensures that the exciter is working properly at the same time.

The admission and exhaust valves (steam inlet and outlet doors in the cylinders) are activated by two camshafts rotated by gears located on the reluctor axis.

A speed regulator with flyweights maintains the correct speed by acting on the opening of the admission valves of the high-pressure cylinder (which allows steam output to be regulated). Following its work in the four cylinders, the steam passes into two surface condensers which will cool it in the first instance.

The condensers are two cylinders each containing 300 pipes covered by cold water. When in contact with these cold pipes, the steam is transformed into warm water, called condensation, which is sucked in by two vacuum pumps, where a cold water sprinkling system carries out a second condensation. All this mechanical engineering of 170 tons is lubricated by about ten different systems including 48 continuous drip lubricators, oil pumps activated by camshafts.

.., as well as by a manual system using oil can lubrication (315 different points). This engine was bought by the D.M.C Mulhouse company in 1901 and was in use for 46 years. After 30 years of inactivity, 20 000 hours of restoration work were necessary so that you can now see it operating every day.
© 2008 Musée EDF Electropolis, l'aventure de l'électricité - Mulhouse